Where has the OP said he got Sonar 7 from the internet?
That's great that you can run BFD3 in full detail leadfoot, slightly surprising but great ;-)
To the OP, is sonar using your sound cards ASIO driver and what is your sound card?
As Noynekker said, giving your full system specs is always a good idea, mine (like many others) are in my signature so people don't need to ask.
Insufficient RAM is more likely to produce stuttering playback than crashes so I don't think it's a RAM issue.
I have a windows 7 32bit machine with sonar 8.5 and BFD3 runs fine.
You might try downloading the demo of sonar platinum to see if BFD3 runs better.
Steve